1. Field of the Invention
The present invention relates to a vehicle running range switching device of the so-called shift-by-wire type in which a vehicle driver selects the running range (for example, P, R, N, or D) using a shift lever, for example, to output an electric signal.
2. Description of the Related Art
Generally, the running range of a vehicle provided with an automatic transmission is selected by operation of a shift lever so as to shift a manual valve, thereby changing a hydraulic path in a hydraulic controller. One conventional system for execution of the selected shift is a shift-by-wire (SBW) system in which the selected running range is set by electric signals (see, for example, Japanese Patent Laid-Open Hei 7-310820), instead of mechanically actuated by a wire or rod.
Other examples include Japanese Patent Application Laid-Open No.2002-310295which discloses a shift-by-wire (SBW) system that uses a feed screw device, and U.S. Pat. No. 4,843,901 which discloses a shift-by-wire (SBW) system using a worm gear. It is well known that the feed screw device disclosed in Japanese Patent Application Laid-Open No.2002-310295 is capable of achieving a large reduction ratio and high positional accuracy. On the other hand, the worm gear disclosed in U.S. Pat. No. 4,843,901 can also provide a large reduction ratio.
However, the system of Japanese Patent Application Laid-Open No.2002-310295 is configured such that rotation of a motor 2 directly drives, i.e., advances/retracts, a valve rod 11 of a control valve 1 using a feed screw unit 4, which serves as a conversion mechanism. That is, a screw member 42 and the valve rod 11 are disposed coaxially in series, and the screw member 42 is axially moved so as to advance or retract the control rod 11. For this reason, the unit is as long as the sum of the lengths of the screw member 42 and the valve rod 11, plus a length allowing for axial advancement/retraction of the screw member 42 (axial advancement/retraction of the valve rod 11). Consequently, the unit must be substantially enlarged, which has a detrimental effect on freedom of design since positioning of installation is restricted.
As described in Japanese Patent Application Laid-Open No.2002-310295, the rotation of the screw member 42 is stopped by a rod 51 that is connected to a parking unit 5. As a result, the rod 51 must be highly durable. Further, the configuration is such that vibration generated by rotation of a nut member 41 is transmitted directly to the parking unit 5 and the valve rod 11 of the control valve 1 through the screw member 42. Accordingly, durability of the control valve 1 is reduced. Further, because the rod 51 and the valve rod 11 of the control valve 1 are required to transmit different pressures and are driven by the same screw member 42, the screw member 42 may become distorted. In this case, balls within the nut member 41 make irregular localized contact, as a result of which it impossible to drive the nut member 41 smoothly, and efficiency is reduced.
On the other hand, with the mechanism disclosed in U.S. Pat. No. 4,843,901, the worm gear is usually subject to substantial contact resistance with an intermeshed gear, and thus needs to receive a larger force from a motor than that provided by a ball screw in order to be rotated. In addition, if driven in reverse, an even larger force is required, i.e., a substantial torque output from the motor. Therefore, if, for example, the running range cannot be changed because a failure occurs in the motor or the worm gear, with the mechanism disclosed in U.S. Pat. No. 4,843,901, manual release is enabled so as to allow forcible change of the running range and that manual release requires only a small force. Accordingly, the worm gear is mechanically separated from the other gears. However, in this case, manual release cannot be achieved easily because it can be executed only after a complicated procedure for separating the gears mechanically. Further, a mechanism for mechanical separation is required, thus enlarging the range switching device and, consequently, production costs thereof are increased. Moreover, because gears are used for torque transmission, manual release cannot be easily executed, and therefore, the position of installation and method for manual release are restricted. Additionally, after the worm gear and the intermeshed gear are separated, they need to be meshed with each other once again afterwards. As a result, the gears can be damaged, which leads to a reduction in the durability of the device. In particular, when only the worm gear needs to be separated, meshing of the worm gear with another gear needs to be adjusted.